The present invention relates to heating and more particularly to the thermal cycling of specimen carriers.
In many fields specimen carriers in the form of support sheets which may have a multiplicity of wells or impressed sample sites, are used for various processes where small samples are heated or thermally cycled.
A particular example is the Polymerase Chain Reaction method (often referred to as PCR) for replicating DNA samples. Such samples require rapid and accurate thermal cycling, and are typically placed in a multi-well block and cycled between several selected temperatures in a pre-set repeated cycle. It is important that the temperature of the whole of the sheet or more particularly the temperature in each well be as uniform as possible.
The individual samples are normally liquid solutions, typically between 1 xcexcl and 200 xcexcl in volume, contained within individual sample tubes or arrays of sample tubes that may be part of a monolithic plate. It is desirable to minimize temperature differentials within the volume of an individual sample, during thermal processing. The temperature differentials that may be measured within a liquid sample increase with increasing rate of change of temperature and may limit the maximum rate of change of temperature that may be practically employed.
Previous methods of heating such specimen carriers have involved the use of attached heating devices such as wire, strip and film elements and Peltier effect thermoelectric devices, or the use of indirect methods where separately heated fluids are directed into or around the carrier
The previous methods of heating suffer from the disadvantage that heat is generated in a heater that is separate from the specimen carrier that is required to be heated.
The thermal energy must then be transferred from the heater to the carrier sheet, which in the case of an attached heater element occurs, through an insulating barrier and in the case of a fluid transfer mechanism occurs by physically moving fluid from the heater to the sheet.
The separation of the heater from the block introduces a time delay or xe2x80x9clagxe2x80x9d in the temperature control loop. That is to say that the application of power to the heating elements does not produce an instantaneous or near instantaneous increase in the temperature of the block. The presence of a thermal gap or barrier between the heater and the block requires the heater to be hotter than the block if heat energy is to be transferred from the heater to the block. Therefore, there is a further difficulty that cessation of power application to the heater does not instantaneously stop the block from increasing in temperature.
The lag in the temperature control loop will increase as the rate of temperature change of the block is increased. This can lead to inaccuracies in temperature control and limit the practical rates of change of temperature that may be used.
Inaccuracies in terms of thermal uniformity and further lag may be produced when attached heating elements are used, as the elements are attached at particular locations on the block and the heat produced by the elements must be conducted from those particular locations to the bulk of the block. For heat transfer to occur from one pan of the block to another, the first part of the block must be hotter than the other.
Another problem with attaching a thermal element, particularly a Peltier effect device, is that the interface between the block and the thermal device will be subject to mechanical stresses due to differences in the thermal expansion coefficients of the materials involved. Thermal cycling will lead to cyclic stresses that will tend to compromise the reliability of the thermal element and the integrity of the thermal interface.
The present invention aims to solve at least some of these problems by applying direct electrical resistance heating to a metallic specimen carrier. Thus the invention provides a method of heating a specimen carrier in the form of a metallic sheet by applying a heating current to said sheet.
The electric current passing through it directly heats the sheet; this removes lags in the temperature control loop. The whole of the sheet can be substantially instantaneously heated.
Preferably the metallic sheet will be of a metal having high thermal conductivity such as copper or silver. Small variations in metal thickness or thermal loading over the area of the sheet may be tolerated if the thermal conductivity of the sheet is high enough to equalise the temperature differences between any localised high or low temperature regions. The level of temperature variation that may be tolerated will depend upon the application, for PCR, applications more than 0.5 C is not tolerable.
Silver is preferred over copper in some circumstances, for example when rapid thermal cycling is to be used, as silver has a lower specific heat capacity than copper and will therefore require less energy to produce any particular temperature change.
The sheet will generally have a thin section in the region of 0.3 mm thickness, say in the range of 0.2-0.5 mm.
The sheet may be in a form where a matrix of sample wells is incorporated in the sheet.
The sheet may have an impressed regular array of wells to form a block and a basal grid or perforated sheet may be attached to link the tips of the wells at their closed ends to form an extremely rigid three-dimensional structure. In some applications the mechanical stiffness of the block is an important requirement. Where a basal grid is used, heating current is also passed through the metal of the grid. The basal grid is preferably made of the same metal as the block.
While the metallic sheet may be a solid sheet of silver (which may have cavities forming wells) an alternative is to use a metallised plastic tray (which may have impressed wells), in which deposited metal forms a resistive heating element.
Another alternative is to electro form a thin metal tray (which again may have impressed wells), and to coat the metal with a bio-compatible polymer.
These measures enable intimate contact to be achieved between the metallic heating element and the bio-compatible sample receptacles. This gives greatly improved thermal performance in terms of temperature control and rate of change of temperature when the actual temperatures of the reagents in the wells is measured.
The plastic trays are conventionally single use disposable items. The incorporation of the heating element into the plastic trays may increase their cost, but the reduction in cycling time for the PCR reaction more than compensates for any increased cost of the disposable item.
The bottom of the composite tray should be unobstructed if fan cooling is employed. If sub-ambient cooling is required at the end of the PCR cycles, either with a composite tray or a block, chilled liquid spray-cooling may be employed. The boiling point of the liquid should be below the low point of the PCR cycle so that liquid does not remain on the metal of the tray or block to impede heating. This also allows for the latent heat of evaporation of the liquid to increase the cooling effect.
The heating current may be an alternating current supplied by a transformer system wherein the heating power is controlled by regulating the power supplied to the primary winding of the transformer. The sheet to be heated may be made part of the transformer secondary circuit. The secondary winding may be a single or multiple loop of metal that is connected in series with the sheet. By these means, the high current, low voltage power that is required to heat the highly conductive sheet may be simply controlled by regulating the high voltage, low current power supplied to the primary winding of the transformer.
The transformer may comprise a toroidal core having an appropriate mains primary winding and a single bus bar looped through the core and connected in series with the metallic sheet to form a single turn secondary circuit.
When heating samples in sample wells of a carrier in the way described above it is sometimes desirable to provide agitation or stirring.
In a development of the invention the heating current is an alternating current, and a magnet is loosely contained within at least one well and is arranged to be agitated by the alternating current so as to provide a stirring action during the heating.
According to another aspect of the invention there is provided a method of heating a specimen carrier in the form of a metallic sheet and in which a matrix of sample wells is incorporated in the sheet,
which method includes applying an alternating current to said sheet to provide heating of the samples in the wells, and
a magnet is loosely contained within at cast one well and is arranged to be, agitated by the alternating current so as to provide a stirring action during the heating.
Usually, but not necessarily always, each well will contain a magnet.
The sample wells may incorporate samples directly or may carry sample pots or test tubes shaped to closely fit within the wells.
Generally the sample wells may be conical in shape. This helps any stirring action of each magnet within the respective well.
More specifically, in direct resistance heating using alternating current, an oscillating magnetic field is produced at each well by the heating current. A small bar magnet, (typically 5 mm long by 1 mm diameter), may be placed in each sample tube and the heating current will cause oscillating forces to be applied to the magnet. The geometry of the conical section of the sample tube will then constrain the bar to spin about an axis that is not coaxial with, or normal to, the axial dimension of the bar. The stirring action is then similar to that which would be produced by vigorously stirring each individual tube with a manual stirring rod.
The magnets may be made of readily available materials, in particular hard magnetic alloys such as Alnico 4. Rare earth magnets (for example iron-neodymium-boron or samarium-cobalt) may also be used. To prevent contamination of the liquid sample, the magnet may be given an inert coating. Such a coating may be of a bio-compatible polymer such as polypropylene or polycarbonate, or a noble metal such as gold. A noble metal coating has the advantage that it adds no significant volume to the magnet when applied in a coating of sufficient thickness to ensure that the coating is not porous. When using gold a 5 xcexcm thickness is sufficient to provide a porefree coating, and adds a volume of 0.08 xcexcl to the magnet.
The magnets cost much less than the typical reagent mix to be placed in a sample tube, and may therefore be regarded as consumable items. However the magnets may clearly be easily sorted from the waste reagents for cleaning and re-use.
The magnets may be small. In particular embodiments, for a 100 xcexcl liquid sample, a magnet 1 mm in diameter and 5 mm long may be employed. Such a magnet has a volume of 3.9 xcexcl. A 0.5 mm diameter by 3 mm long magnet may be provided for use in smaller tubes and would have a volume of 0.58 xcexcl. The approximate masses of these magnet examples would be 31 mg and 4.5 mg respectively.
In certain embodiments, a magnet is placed in each of the wells to be agitated. In standard practice the shape of the individual wells is conical and the magnet length is chosen such that the long axis of the bar magnet is constrained to be within a range of between 5 and 30 degrees of the axis of the well. Such orientation ensures that the agitation magnet will spin eccentrically and will not jam in the well. The diameter of the magnet should be as small as is practical, in order to minimise the volume of the magnet. The passage of the alternating heating current through the block gives rise to an alternating magnetic field circling the block in a plane normal to the direction of current flow. The alternating magnetic field causes alternating forces to be applied to the bar magnets as they try to align themselves with the magnetic field. The conical shape of the wells constrains the movement of the magnets, which then spin eccentrically in each well.
The effect of the eccentric spinning of the magnets is to vigorously stir the liquid sample in each of the wells to which a magnet has been introduced. The stirring effect almost completely eliminates any of the temperature differentials that may be observed in a static sample during thermal cycling.
Preferably, the bottom of the sheet, even if a basal grid is attached, has an open structure with a large surface area. Such a surface is ideal for forced-air cooling. Moreover, preferably there are no attached elements to impede free and full contact between the metal of the sheet and moving air.
Ducting of the air may be provided to encourage even cooling effects over the extent of the sheet. To allow for controlled cooling rates, the air movement may be under proportional control. The control response time of a device that imparts movement to air, for instance a mechanical element such as a fan, is slow compared to the fast electronic control response of the heating system. The heating system may therefore be used together with the fan to control the temperature changes of the sheet during cooling
The secondary winding in series with the sheet may have more than one loop through the core of the transformer.
The power supply means and control for the heating current may be a high frequency AC power supply permitting a reduction in the amount of material in the transformer core.
The thermal uniformity of the sheet will be dependent on the heating power dissipation at any point in the sheet being matched to the thermal characteristics of that point. For instance, a point around the centre of the sheet will be surrounded by temperature controlled metal, whereas a point at the edge of the sheet or block will have temperature controlled metal on one side and ambient air on the other. The geometry of the sheet may be adjusted with the aim of achieving thermal uniformity. In general practice the geometry of sample sites or wells of a sheet or block will be a standardised regular array. The industry standard arrays consist of 48, 96 or 384 wells in a 110xc3x9775 mm rectangular plate or block. These layouts are arbitrary and larger arrays of 768 and 1536 wells are appearing.
Typically, the geometric factors that may be varied comprise the thickness of the metal from which the sheet is formed, and if a basal grid is used, the geometry of the webs in the plane of the grid.